Make Stanza.appendErrorIfExists() aware of the enclosing XML namespace

Originally discovered by Paul Schaub:

Fixes an inconvenience, where an IQ with the implicit namespace
`jabber:client` would append the namespace to an error child element
like this:

```
<iq (xmlns='jabber:client) <!-- in parenthesis since the NS is implicit --> ... >
    <error xmlns='jabber:client' <!-- this NS is too much --> ... />
</iq>
``

smack-core/src/main/java/org/jivesoftware/smack/packet/IQ.java

@@ -137,7 +137,7 @@ public abstract class IQ extends Stanza {
             buf.attribute("type", type.toString());
         }
         buf.rightAngleBracket();
-        buf.append(getChildElementXML());
+        buf.append(getChildElementXML(enclosingNamespace));
         buf.closeElement(IQ_ELEMENT);
         return buf;
     }
@@ -149,10 +149,22 @@ public abstract class IQ extends Stanza {
      * @return the child element section of the IQ XML.
      */
     public final XmlStringBuilder getChildElementXML() {
+        return getChildElementXML(null);
+    }
+
+    /**
+     * Returns the sub-element XML section of the IQ packet, or the empty String if there
+     * isn't one.
+     *
+     * @param enclosingNamespace the enclosing XML namespace.
+     * @return the child element section of the IQ XML.
+     * @since 4.3.0
+     */
+    public final XmlStringBuilder getChildElementXML(String enclosingNamespace) {
         XmlStringBuilder xml = new XmlStringBuilder();
         if (type == Type.error) {
             // Add the error sub-packet, if there is one.
-            appendErrorIfExists(xml);
+            appendErrorIfExists(xml, enclosingNamespace);
         }
         else if (childElementName != null) {
             // Add the query section if there is one.

smack-core/src/main/java/org/jivesoftware/smack/packet/Message.java

@@ -501,7 +501,7 @@ public final class Message extends Stanza implements TypedCloneable<Message> {
         buf.optElement("thread", thread);
         // Append the error subpacket if the message type is an error.
         if (type == Type.error) {
-            appendErrorIfExists(buf);
+            appendErrorIfExists(buf, enclosingNamespace);
         }
 
         // Add extension elements, if any are defined.

smack-core/src/main/java/org/jivesoftware/smack/packet/Presence.java

@@ -292,7 +292,7 @@ public final class Presence extends Stanza implements TypedCloneable<Presence> {
         buf.append(getExtensions(), enclosingNamespace);
 
         // Add the error sub-packet, if there is one.
-        appendErrorIfExists(buf);
+        appendErrorIfExists(buf, enclosingNamespace);
 
         buf.closeElement(ELEMENT);
 

smack-core/src/main/java/org/jivesoftware/smack/packet/Stanza.java

@@ -541,10 +541,10 @@ public abstract class Stanza implements TopLevelStreamElement {
      *
      * @param xml the XmlStringBuilder to append the error to.
      */
-    protected void appendErrorIfExists(XmlStringBuilder xml) {
+    protected void appendErrorIfExists(XmlStringBuilder xml, String enclosingNamespace) {
         StanzaError error = getError();
         if (error != null) {
-            xml.append(error.toXML());
+            xml.append(error.toXML(enclosingNamespace));
         }
     }
 }